The positive photoresist composition of the present invention is coated on a substrate such as a semiconductor wafer, glass, ceramic or metal in a thickness of from 0.5 to 2 .mu.m by a spin coating method or a roller coating method. Subsequently, the coated layer is heated and dried, and a circuit pattern or other pattern is printed on the layer by, for example, irradiation with ultraviolet ray through an exposure mask, and then the exposed photoresist layer is subjected to baking after exposure, if necessary, and is developed to form a positive image.
Further, the substrate can be etched using the positive image as a mask to form the pattern on the substrate. Typical applications of the positive photoresist are manufacture of semiconductors such as IC and the like, manufacture of circuit boards for liquid crystals and thermal heads, and other photofabrication processes.
Positive photoresist compositions generally comprise an alkali-soluble resin binder such as a novolak resin and the like and a naphthoquinonediazide compound as a photosensitive material.
Novolak resins used as a binder are soluble in an aqueous alkaline solution without swelling therein and are highly resistant particularly to plasma etching when the image formed is used as a mask for etching. They are, therefore, particularly useful in this application. On the other hand, naphthoquinonediazide compounds used as a photosensitive material act themselves as a dissolution inhibitor to lower the alkali solubility of the novolak resin, but are peculiar in that when decomposed by light irradiation to produce an alkali-soluble substance which elevates the alkali solubility of the novolak resin. Because of the great light dependent variation of the properties, naphthoquinonediazide compounds are particularly useful as a photosensitive material in a positive photoresist composition.
Hitherto, various positive photoresists comprising a novolak resin and a naphthoquinonediazide photosensitive material have been developed and put into practical use from such a point. In particular, the progress of the resist materials is conspicuous in high resolving power and the materials have attained sufficient results in forming a line width of as small as sub-micrometers.
Conventionally, it has been thought that resists having a high contrast (gamma value (.gamma.)) are advantageously used to elevate the resolving power to obtain image reproduction of a good pattern form. Therefore, researches have been made to develop a resist composition serving such a purpose. Many publications disclosing such a technique have been published. In particular, with respect to the novolak resin as a main component of positive photoresists, many patent applications have been filed based on the monomer component, distribution of molecular weight, the synthesis method and the like, and some results have been obtained. On the other hand, with respect to another main component photosensitive material, compounds having various structures which are seemed to be advantageous to realize a high contrast have been disclosed. By designing a positive photoresist using these techniques, it has become possible to develop a resist having an ultrahigh resolving power capable of resolving the pattern of the same degree of the dimension as the wavelength of light.
However, integrated circuits have added to the degree of integration increasingly, and the formation of ultrafine patterns of a line width of 0.5 .mu.m or less has been required in the production of a semiconductor substrate such as super LSI. In such an application, photoresists having a broad development latitude are required, in particular, to obtain a stable high resolution and secure the formation of a constant line width. Further, it is required that resist residues are not generated on the pattern of the resist after development to prevent processing defects of the circuit.
Moreover, it has been found that in the formation of, in particular, ultrafine patterns of a line width of 0.5 .mu.m or less, for example, even if a certain level of resolving power can be obtained at a certain coating layer thickness, the resolving power deteriorates with an extremely trace variation of the coating layer thickness (hereinafter, referred to as "layer thickness reliance"). A resolving power is largely changed with the variation of the layer thickness of only several hundredths of a micrometer, and it has been found that almost all the representative positive photoresists now commercially available have more or less such a tendency. Specifically, when the layer thickness of the resist before exposure changes in the range of .lambda./4n based on the prescribed layer thickness (where .lambda. is an exposure wavelength and n is a refractive index of the resist layer at that wavelength), the resolving power to be obtained fluctuates correspondingly.
The presence of this layer thickness reliance is disclosed, for example, in SPIE Proceedings, Vol. 1925, page 626 (1993) such that this phenomenon is caused by the multiple reflection effect of light in a resist layer.
It has been found that, in particular, when the contrast of a resist is to be heightened to obtain a high resolving power and a resist pattern having a rectangle cross-section, this layer thickness reliance often becomes large. When a semiconductor substrate is practically processed, a resist pattern is formed using a resist layer of a delicately different coated layer thicknesses according to surface roughness of the substrate or coating unevenness. Accordingly, this layer thickness reliance has been an obstacle when an ultrafine pattern near to the limitation of the resolving power using a positive photoresist is processed.
Hitherto, various 1,2-naphthoquinonediazide compounds of polyhydroxy compounds having a specific structure have been proposed to heighten a resolving power, for example, those disclosed in JP-A-57-63526 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), JP-A-60-163043, JP-A-62-10645, JP-A-62-10646, JP-A-62-150245, JP-A-63-220139, JP-A-64-76047, JP-A-1-189644, JP-A-2-285351, JP-A-2-296248, JP-A-2-296249, JP-A-3-48249, JP-A-3-48250, JP-A-3-158856, JP-A-3-228057, JP-A-4-502519, U.S. Pat. No. 4,957,846, 4,992,356, 5,151,340, 5,178,986, and European Patent 530148. However, these photosensitive materials have not been sufficient to lower the layer thickness reliance.
1,2-Naphthoquinonediazidosulfonates of the compound represented by the following formula (II) are disclosed, for example, in JP-A-2-296248, as the photosensitive material having four aromatic rings in the molecule, but these photosensitive materials are not sufficient for lowering the layer thickness reliance. ##STR2##
Further, 1,2-naphthoquinonediazidosulfonates of the compounds represented by the following formula (III) and formula (IV) are disclosed, for example, in JP-A-3-291250 and JP-A-6-19130, respectively, as the photosensitive material having a cycloalkyl group in the molecule, but these photosensitive materials are not sufficient for lowering the layer thickness reliance. ##STR3##
On the other hand, resists having a high contrast and a high resolving power can be obtained by using a photosensitive material having a hydroxyl group in the molecule as disclosed, for example, in JP-B-37-18015 (the term "JP-B" as used herein means an "examined Japanese patent publication"), JP-A-58-150948, JP-A-2-19846, JP-A-2-103543, JP-A-3-228057, JP-A-5-323597, JP-A-6-148878, JP-A-6-167805, JP-A-6-202321, U.S. Pat. Nos. 3,061,430, 3,130,047, 3,130,048, 3,130,049, 3,102,809, 3,184,310, 3,188,210, 3,180,733, West German Patent 938,233, SPIE Proceeding, Vol. 631, page 210, ibid., Vol. 1672, page 231 (1992), ibid., Vol. 1672, page 262 (1992), and ibid., Vol. 1925, page 227 (1993).
The present inventors have proposed compounds represented by the following formulae (V) and (VI) as selectively esterified photosensitive materials having four or five aromatic rings in the molecule (JP-A-3-228057). ##STR4##
However, these compounds are not- sufficient for lowering the layer thickness reliance, either.
Thus, it has not been known absolutely how to design the resist composition to lower the layer thickness reliance and obtain a high resolving power irrespective of the variation of the layer thickness.
Further, according to the increase of the degree of integration in semiconductor devices, demands for particles of the positive photoresist have increased year after year. As there is a so-called 1/10 rule in the art of the semiconductor, a particle size of 1/10 or more of the minimum line width of a device affects the yield (e.g., Ultraclean Technology, Vol. 3, No. 1, page 79 (1991)).
Various contrivances have been usefully adopted to reduce these particles such that an ultrafine filter having a pore diameter of 0.1 .mu.m or 0.05 .mu.m is used at the time of producing a resist.
However, even if there are less such particles in the resist at the time of resist production, they often increase with the lapse of time almost because of 1,2-quinonediazide photosensitive material and various means have been taken to improve the property with the lapse of time.
Various methods have been tried hitherto, for example, the method of using such a photosensitizer as a part of the hydroxyl group in the polyhydroxy compound is acylated or sulfonylated (JP-A-62-178562), the method of using a mixture of 1,2-naphthoquinonediazido-4-sulfonate and 1,2-naphthoquinone-diazido-5-sulfonate (JP-A-62-284354), the method of using a thermal modified 1,2-naphthoquinonediazide photosensitizer (JP-A-63-113451), the method of reducing the remaining catalyst of a photosensitizer (JP-A-63-236030), the method of synthesizing a photosensitizer in the presence of an anion exchange resin (JP-A-63-236031), and the method of mixing a solvent having good solubility with a photosensitizer (JP-A-61-260239 and JP-A-1-293340).